Fuel cells are increasingly being used as a power source for electric vehicles and other applications. An exemplary fuel cell has a membrane electrode assembly (MEA) with catalytic electrodes and a proton exchange membrane (PEM) formed between the electrodes. Gas diffusion media play an important role in PEM fuel cells. Generally disposed between catalytic electrodes and flow field channels in the fuel cell, they provide reactant and product permeability, electronic conductivity, and heat conductivity, as well as mechanical strength needed for proper functioning of the fuel cell.
During operation of the fuel cell, water is generated at the cathode based on electrochemical reactions involving hydrogen and oxygen occurring within the MEA. Efficient operation of a fuel cell depends on the ability to provide effective water management in the system. For example, the diffusion media prevent the electrodes from flooding (i.e., filling with water and severely restricting O2 access) by removing product water away from the catalyst layer while maintaining reactant gas flow from the bipolar plate through to the catalyst layer.
The gas diffusion media are generally constructed of carbon fiber containing materials. Although carbon fibers are themselves relatively hydrophobic, it is usually desirable to increase the hydrophobicity or to at least treat the carbon fiber with a more stable hydrophobic coating. Adding a hydrophobic agent such as polytetrafluoroethylene (FTFE) to the carbon fiber diffusion media is a common process for increasing the hydrophobicity. This process is normally done by dipping carbon fiber papers into a solution that contains PTFE particles and other wetting agents, such as non-ionic surfactants.
Fuel cell stacks can contain a large number of fuel cells depending on the power requirement of the application. For example, typical fuel stacks have up to 200 individual fuel cells and more. Because the fuel cells in the stacks operate in series, a weakness or poor performance in one cell can translate into poor performance of the entire stack. For this reason, it is desirable for every fuel cell in the stack to operate at high efficiency.
Because typical fuel stacks contain so many individual fuel cells, it has been observed that, even with a high degree of reliability of manufacture of diffusion media, it is sometimes observed that an individual or several diffusion media will have less than optimum performance, especially at a high relative humidity. When that occurs, a fuel stack containing such a fuel cell will generally exhibit less than optimum performance. Thus, diffusion media with enhanced hydrophobic properties and methods for producing them that lead to consistent results among hundreds of fuel cells in a single fuel stack would be an advance in the art.